1. Field of the Invention
The present invention relates to a positive electrode active material for nonaqueous electrolyte secondary batteries, a method for manufacturing the same, and a nonaqueous electrolyte secondary battery using said positive electrode active material.
2. Description of the Related Art
In recent years, with the spread of portable electronic equipment, such as cell phones and notebook-sized personal computers, development of a small and lightweight nonaqueous electrolyte secondary battery having a high energy density has been strongly desired. Also, development of a high-output secondary battery as a battery for electric vehicles, such as hybrid vehicles, has been strongly desired.
As a secondary battery satisfying such demands, there is a lithium-ion secondary battery. This lithium-ion secondary battery comprises a negative electrode, a positive electrode, an electrolyte solution, and the like, and, as active materials of the negative electrode and the positive electrode, materials capable of desorption and insertion of lithium are used.
At present, research and development of such lithium-ion secondary battery are being actively conducted, and particularly, since a 4V class high voltage can be achieved by a lithium-ion secondary battery using lithium metal composite oxide having a layered or spinel structure as a positive electrode material, the commercialization thereof as a battery having a high energy density is progressing. As a material which has been mainly proposed until now, it may include lithium-cobalt composite oxide (LiCoO2), which is relatively easily synthesized; lithium-nickel composite oxide (LiNiO2), in which nickel, more inexpensive than cobalt, is used; lithium-nickel-cobalt-manganese composite oxide (LiNi1/3Co1/3Mn1/3O2); lithium-manganese composite oxide (LiMn2O4), in which manganese is used; and the like.
Among these, lithium-nickel composite oxide and lithium-nickel-cobalt-manganese composite oxide have been highlighted as a material having excellent cycle characteristics and providing high output with low resistance, and, in recent years, low resistance, which is necessary for achieving high output, has been regarded as important.
As a method for achieving the low resistance, addition of different elements is applied, and particularly, it is supposed that a high-valent transition metal, such as W, Mo, Nb, Ta, or Re, is useful.
For example, Japanese Patent Application Laid-Open No. 2009-289726 proposes a lithium transition metal compound powder for use as a positive electrode material for lithium secondary batteries, the lithium transition metal compound powder containing 0.1 to 5 mol % of at least one element selected from the group consisting of Mo, W, Nb, Ta, and Re, with respect to a total molar amount of Mn, Ni, and Co, and describes that an atomic ratio of a total of Mo, W, Nb, Ta, and Re to a total of metallic elements other than Li and Mo, W, Nb, Ta, Re, on the surface of primary particles, is preferably five or more times than the atomic ratio of the whole of the primary particles.
According to this proposal, there can be attempted low cost, high safety and high load characteristics, as well as improvement in powder handling, in the lithium transition metal compound powder for use as a positive electrode material for lithium secondary batteries.
However, the above-mentioned lithium transition metal compound powder is obtained in such a manner that a raw material is pulverized in a liquid medium, a slurry in which the raw material is uniformly dispersed is then sprayed and dried to be burned. Therefore, there is a problem that a part of the different element, such as Mo, W, Nb, Ta, or Re, substitutes for Ni which has been arranged in layers, whereby battery characteristics, such as battery capacity and cycle characteristics, decrease.
Japanese Patent Application Laid-Open No. 2005-251716 proposes a positive electrode active material for nonaqueous electrolyte secondary batteries which has at least lithium transition metal composite oxide having a layered structure, wherein the lithium transition metal composite oxide exists in a form of particles comprising either or both of primary particles and secondary particles composed of aggregation of the primary particles, and has, at least on the surface of the particles thereof, a compound comprising at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine.
Thus, Japanese Patent Application Laid-Open No. 2005-251716 provides a positive electrode active material for nonaqueous electrolyte secondary batteries, which shows excellent battery characteristics even under severer environment conditions for use, and particularly, describes the improvement of initial characteristics without any improvement in thermal stability, load characteristics, and output characteristics since the positive electrode active material has, on the surface of the particles thereof, a compound comprising at least one selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine.
However, the effect of at least one kind of addition element selected from the group consisting of molybdenum, vanadium, tungsten, boron, and fluorine exists in improvement in initial characteristics, that is, initial discharge capacity and initial efficiency, and thus it does not refer to output characteristics. According to the disclosed manufacturing method, there is a problem that the addition element is mixed and burned with a hydroxide which has been heat-treated simultaneously with a lithium compound and a part of the addition element therefore substitutes for nickel which has been arranged in layers, whereby battery characteristics decrease.
Furthermore, Japanese Patent Application Laid-Open No. H11-16566 proposes a positive electrode active material which is coated with a metal containing at least one selected from the group consisting of Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo and/or an intermetallic compound obtained by a combination of a plurality of the above mentioned elements, and/or oxide.
Japanese Patent Application Laid-Open No. H11-16566 describes that such coating enables oxygen gas to be absorbed and safety to be secured, but does not disclose output characteristics at all. Moreover, in the disclosed manufacturing method, the coating is performed using a planetary ball mill, and such coating method gives physical damages to the positive electrode active material, and causes decrease in battery characteristics.
Japanese Patent Application Laid-Open No. 2010-40383 proposes a positive electrode active material having a carbonate ion content of not more than 0.15 wt %, which is obtained in such a manner that a tungstate compound is adhered to composite oxide particles composed mainly of lithium nickelate and heat-treatment is performed.
According to this proposal, the presence of a tungstate compound or a decomposed material of a tungstate compound on a surface of the positive electrode active material allows oxidation activity on a surface of the composite oxide particles in a charge state to be controlled, whereby gas generation by decomposition of a nonaqueous electrolyte solution or the like can be controlled, however, output characteristics are not disclosed at all.
Moreover, in the disclosed manufacturing method, a solution obtained by dissolving a sulfate compound, a nitrate compound, a borate compound, or a phosphate compound in a solvent as an adhering component, together with a tungstate compound, is preferably adhered to the composite oxide particles heated beyond the boiling point of the solution with the adhering components dissolved therein, and the solvent is removed in a short time, and therefore, the tungsten compound is not fully dispersed on a surface of the composite oxide particles, and it is hard to say that the tungstate compound is uniformly adhered.
In view of such problems, the present invention aims to provide a positive electrode active material for nonaqueous electrolyte secondary batteries which achieves high output as well as high capacity when used as a positive electrode material.